EP0799714B1

EP0799714B1 - Thermal transfer sheet

Info

Publication number: EP0799714B1
Application number: EP19970302267
Authority: EP
Inventors: Koichi Dai Nippon Printing Co. Ltd. Nakamura
Original assignee: Dai Nippon Printing Co Ltd
Current assignee: Dai Nippon Printing Co Ltd
Priority date: 1996-04-03
Filing date: 1997-04-02
Publication date: 2001-11-14
Anticipated expiration: 2017-04-02
Also published as: US6210794B1; DE69708186D1; DE69708186T2; EP0799714A1; US20010000740A1

Description

The present invention relates to a thermal transfer sheet to be used for a thermal transfer printer utilizing a heating means such as a thermal head or a laser, and more specifically to a thermal transfer sheet which provides good printing quality in thermal printing in full colour and has an excellent colour reproduction property, and is therefore adapted to be used for a poster or an information board. Preferably, such a thermal transfer sheet has improved weatherproofing properties, wear resistance and chemical resistance properties, so that a plastic substrate as an image receiving sheet to which an image has thermally been transferred with the use of the thermal transfer sheet is adapted to be used for an open-air article such as a license plate for a vehicle such as a car, a road sign, or the like.
There has conventionally been known a fusion transfer method in which a colouring agent is transferred to a image receiving sheet such as paper or a plastic sheet by impressing energy corresponding to image information by means of a heating device such as a thermal head with the use of a thermal transfer sheet obtained by carrying a colouring layer or a heat fusible colouring ink layer in which colouring agents such as pigment and dye are dispersed in a binder such as heat fusible wax or resin, on a substrate sheet such as a plastic film.
The transferred image formed by this fusion transfer method has high density and is excellent in clarity, and is therefore adapted to record a binary image such as a character or a line drawing. It is also possible to form a polychrome or colour image with the use of decreased kinds of colour by making a multiple printing record of colouring layers or heat fusible colouring ink layers on an image receiving sheet by using a thermal transfer sheet having the heat fusible colouring ink layers of yellow, magenta, cyan and the like.
However, there have been many conventional thermal transfer sheets having the colouring layer, in which a wax was used as a binder of the colouring layer , and such conventional thermal transfer sheets had a remarkable problem that image-printed material obtained thereby was poor in wear resistance.
In view of this problem, another thermal transfer sheet has been prepared using a resin as the binder of the colouring layer. When a multiple printing of the colouring layer was conducted with the use of such a kind of the thermal transfer sheet, there was however caused a problem of incomplete printing, i.e., a void or a printing defect on an overlapped portion of the colouring layers.
In the conventional thermal transfer sheet having the heat fusible colouring ink layer, the colouring agents used therein, especially pigments of yellow and magenta do not have a high weatherproofing property. As a result, the indoor normal use of the thermal transfer sheet for materials such as a leaflet or a brochure causes no problem, whereas the outdoor use thereof with its exposure to direct sunlight cause a problem of fading.
The first object of the present invention is therefore to provide a thermal transfer sheet which permits to solve the first of the above-mentioned problems with the result that a printed material obtained by the thermal printing with the use of the thermal transfer sheet can have a good printing quality without the occurrence of the void and the printing defect on an overlapped portion of the colouring layers, and has an excellent colour reproduction property in full colour.
A secondary object of the present invention is to provide in preferred embodiments a thermal transfer sheet in which a printed material obtained by the thermal printing with the use of the thermal transfer sheet has an excellent weatherproofing property, thus permitting the production of a colour image without the occurrence of change such as fading even when the printed material is applied to the outdoor use.
The thermal transfer sheet of present invention for attainment of the aforementioned first object comprises a substrate sheet, a release layer formed on one surface of said substrate sheet, a colouring layer formed on said release layer and an adhesive layer formed on said colouring layer, wherein said release layer and said adhesive layer are formed such that each contains a material of the same kind and said colouring layer has multiple areas each consisting essentially of a respective colouring agent and a resin but without wax.
According to the above-mentioned thermal transfer sheet of the present invention, by forming the release layer, the colouring layer and the adhesive layer in this order on the one surface of the substrate sheet, it is possible to impart the releasability, colouring property and adhesivity provided by these layer to the thermal transfer sheet so as to carry out effectively these functions. A multiple printing is often conducted with the use of the thermal transfer sheet by carrying out the first thermal transfer step to transfer the first set of the adhesive layer, the colouring layer and the release layer in this order onto the surface of an image receiving sheet and then carrying out the second thermal transfer step to transfer the second set of the adhesive layer, the colouring layer and the release layer in this order onto the release layer of the first set so as to bring the release layer of the first set into contact with the adhesive layer of the second set. When such a multiple printing is conducted, the release layer of the first set and the adhesive layer of the second set which are brought into contact with each other, are fused to form the mixture thereof by heat for the thermal transfer printing, thus improving the adhesivity of the release layer and the adhesive layer and the reproducibility of printed characters in the form of dots, since the release layer and the adhesive layer contain a material of the same kind.
The aforementioned same kind of material may be carnauba wax, polyethylene wax or polyester resin.
The above-mentioned colouring layer may comprise a heat fusible colouring ink layer; and a benzimidazolone-mono-azo organic pigment may be used as a yellow colouring agent in said heat fusible colouring ink layer, a mixture of quinacridone-mono-azo and benzimidazolone-mono-azo organic pigments may be used as a magenta colouring agent in said heat fusible colouring ink layer, and a phthalocyanine organic pigment may be used as a cyan colouring agent in said heat fusible colouring ink layer.
The resin is used as a binder in said heat fusible colouring ink layer.
By using in said heat fusible colouring ink layer the benzimidazolone-mono-azo organic pigment as a yellow colouring agent, the mixture of quinacridone-mono-azo and benzimidazolone-mono-azo organic pigments as a magenta colouring agent and the phthalocyanine organic pigment as a cyan colouring agent, it is possible to impart an excellent weatherproofing property to a printed material obtained by the thermal printing with the use of the thermal transfer sheet, thus permitting the outdoor use of the printed material, unlike a printed material obtained by the thermal printing with the use of the conventional thermal transfer sheet, which cannot be applied to such an outdoor use.
The invention will be further described and illustrated with reference to the accompanying drawings in which:-
Figure 1 is a schematic partial cross sectional view illustrating the thermal transfer sheet of the first embodiment of the present invention for attaining the first object; and
Figure 2 is a schematic partial cross sectional view illustrating the thermal transfer sheet of the second embodiment of the present invention for attaining the first object.
Now, the thermal transfer sheet of the first embodiment of the present invention for attaining the first object will be described in detail with reference to Figure 1. Figure 1 is a schematic partial cross sectional view illustrating the thermal transfer sheet of the aforementioned first embodiment of the present invention.
As shown in Figure 1, the thermal transfer sheet A1 of the first embodiment of the present invention comprises a substrate sheet 1, a release layer 3 formed on one surface of the substrate sheet 1, a colouring layer 2 formed on the release layer 3 and an adhesive layer 4 formed on the colouring layer 2.
Description will be given below of the substrate sheet 1, the colouring layer 2, the release layer 3 and the adhesive layer 4.

[Substrate sheet 1]

As the substrate sheet 1 used in the thermal transfer sheet A1, the same substrate sheet as that used in the conventional thermal transfer sheet may per se be used. There is however no specific restriction thereto.
The preferable example of the substrate sheet 1 may include polyester, polypropylene, cellophane, polycarbonate, cellulose acetate, polyethylene, polyvinyl chloride, polystyrene, nylon, polyimide, polyvinylidene chloride, polyvinyl alcohol, fluorine resin, chlorinated rubber, plastic such as ionomer, paper such as condenser paper or paraffin paper, or nonwoven fabric. The substrate sheet 1 may be formed in composite form of these materials. Although the thickness of the substrate sheet 1 may be optionally changed in accordance with a material to be used so as to provide suitable density and heat conductivity, it is preferably 2 to 25 µm, for example.

[Colouring layer 2]

In the colouring layer 2 of the thermal transfer sheet A1 of the present invention, there is applicable an ink layer having a plurality of colours in which at least two kinds of ink of yellow, magenta, cyan and black are applied onto the substrate sheet 1 in the width direction or the longitudinal direction thereof. In each area, the respective ink layer comprises a colouring agent and a resin binder. Various kind of additives, for example a dispersion agent and an antistatic agent may be added as an occasion demands. A multiple printing can be conducted with the use of the thermal transfer sheet A1 of the present invention.
The colouring agents of yellow, magenta, cyan, black, etc. used in the present invention can properly be selected from the conventional dyes and pigments. The preferable coloring agents may include organic pigments of benzimidazolone-mono-azo, quinacridone, phthalocyanine, threne, dioxazine, isoindolinone, perylene, thioindigo, pyrrocoline, fulorpine and quinophthalone. These organic pigments have an excellent weatherproofing property, thus causing no occurrence of fading even when a printed material with the use of them is applied to an outdoor use such as a poster or an information board.
The binder used in the colouring agent 2 is a resin. The representative examples of the resin may include thermoplastic elastomer such as cellulose resin, melamine resin, polyester resin, polyamide resin, polyolefin resin, acrylic resin, styrene resin, ethylene-vinyl acetate copolymer, styrene-butadiene rubber and the like. It is preferable to use the resin having a relatively low softening point of 50 to 80°C which has conventionally been used as a thermo-sensitive adhesive agent. Of the resins used as the binder, the cellulose resin, melamine resin and acrylic resin are preferably used in view of the transferring property, the wear resistance, the heat resistance and the like.
It is preferable to use an ink composition comprising 90 to 20 wt.% of the colouring agent and 80 to 10 wt.% of the resin in order to form the above-mentioned colouring layer 2. When the colouring agent content is smaller than 20 wt.%, an amount of the applied composition must be increased, thus leading to insufficient printing sensitivity. When the colouring agent content is larger than 90 wt.%, on the other hand, a sufficient film forming property may not be obtained, thus causing the deterioration of the wear resistance of a printed material.
The colouring layer 2 can be formed with the use of the colouring layer forming composition prepared by blending the aforementioned colouring agent and the resin binder, and in addition, a solvent such as water and organic solvent, if necessary, by means of the conventional known method such as a hot melt coating, a hot lacquer coating, a gravure direct coating, a gravure reverse coating, a knife coating, an air coating and a roll coating, so that the colouring layer 2 has a thickness of from 0.05 to 5 µm, preferably of from 0.3 to 1.5 µm in a dry condition.
When the thickness of the dried coating film is under 0.05 µm, there may occur a problem of the film forming property, making it impossible to form a uniform ink layer, thus leading to deterioration of the wear resistance of the printed material. When the thickness thereof is over 5 µm, on the other hand, high energy is required for the thermal transfer printing, with the result that the printing may be conducted only by the specific thermal transfer printer, and the printing sensitivity may tend to be insufficient. [Release layer 3]
In the present invention, the release layer 3 is formed between the substrate 1 and the colouring layer 2. The release layer 3 preferably mainly comprises a wax, and there may be added thereto the thermoplastic elastomer, for example, polyolefin resin, polyester resin or the like, as set forth in the description of the colouring layer 2.
As a typical example of the wax used for the release layer 3, there will be listed up micro-crystalline wax, carnauba wax or paraffin wax. Furthermore, the following waxes may be used: Fischer-Tropsch wax, various kinds of low molecular weight polyethylenes, Japan wax, bees wax, whale wax, insect wax, wool wax, shellac wax, candelilla wax, petrolatum, polyester wax, partially modified wax, fatty acid ester, fatty acid amide, and so on. Of these wax, it is preferable to use the wax having a melting point of 50 to 85°C. With a melting point of up to 50°C, there may be a problem of a storing property. With a melting point of at least 85°C, on the other hand, printing sensitivity may tend to be insufficient.
The thermal transfer sheet A1 is characterized in that the release layer 3 and an adhesive layer 4 described later are formed to contain the same kind of material and to have an excellent adhesivity to plastic material such as polyethylene terephthalate and vinyl chloride. As the above-mentioned same kind of material, there may be listed up the thermoplastic elastomer and the wax which are described above. Of these materials, it is preferably use carnauba wax, polyethylene wax or polyester resin. These materials may be used alone or in combination with each other.
The release layer 3 can be formed with the use of a release layer forming composition, by means of the conventional known method such as a hot melt coating, a hot lacquer coating, a gravure direct coating, a gravure reverse coating, a knife coating, an air coating and a roll coating, so that the colouring layer 2 has a thickness of from 0.05 to 5 µm in a dry condition. When the thickness of the dried film is under 0.05 µm, it is impossible to inhibit the adhesive property of the colouring layer 2 to the substrate sheet 1 [Adhesive layer 4]
According to the thermal transfer sheet A1 of the present invention, it is possible to improve the adhesivity between an image receiving sheet and the colouring layer 2 by forming the adhesive layer 4 on the colouring layer 2. The adhesive layer 4 mainly comprises thermoplastic elastomer which is softened to provide adhesivity by heat of a thermal head , a laser or the like. A blocking preventing agent such as of wax; amide, ester and salt of higher fatty acid; powder of fluoroplastics or inorganic material or the like may be added to the aforementioned elastomer in order to prevent a blocking phenomenon when winding the obtained thermal transfer sheet into a roll. As a typical example of the wax to be added to the elastomer, there will be listed up micro-crystalline wax, carnauba wax or paraffin wax. Furthermore, the following waxes may be used: Fischer-Tropsch wax, various kinds of low molecular weight polyethylenes, Japan wax, bees wax, whale wax, insect wax, wool wax, shellac wax, candelilla wax, petrolatum, polyester wax, partially modified wax, fatty acid ester, fatty acid amide, and so on.
As typical examples of the thermoplastic elastomer, there may be listed ethylene-vinyl acetate copolymer (EVA), ethylene-acrylic acid ester copolymer (EEA), polyester resin, polyethylene, polystyrene, polypropylene, polybutene, petroleum resin, vinyl chloride resin, vinyl chloride-vinyl acetate copolymer, polyvinyl alcohol, vinylidene chloride resin, methacrylate resin, polyamide, polycarbonate, polyvinylformal, polyvinyl butyral, acetylcellulose, nitrocellulose, polyvinyl acetate, polyisobutylene, ethylcellolose, polyacetal and the like. It is preferable to use the elastomer having a relatively low softening point, for example, of 50 to 150°C, which is conventionally used as the thermo-sensitive adhesive agent.
In the thermal transfer sheet A1, the adhesive layer 4 and the release layer 3 are formed so as to have a material of the same kind in common. The aforementioned same kind of material comprises the thermoplastic elastomer described above and wax. Of these materials, polyester resin, carnauba wax or polyethylene wax may preferably be used.
The adhesive layer 4 can be formed with the use of an adhesive layer forming composition which is obtained by dissolving or dispersing the above-mentioned thermoplastic elastomer and additive into a hot-melt coating composition or a proper organic solvent or water, by means of the conventional known method such as a hot melt coating, a hot lacquer coating, a gravure direct coating, a gravure reverse coating, a knife coating, an air coating and a roll coating, so that the adhesive layer 4 has a thickness of from 0.05 to 5 µm in a dry condition. With a thickness of the dried film of under 0.05 µm, there may be caused an inferior adhesivity between the image receiving sheet and the colouring layer 2, thus leading to occurrence of printing defect when conducting the thermal transfer printing. With a thickness thereof of over 5 µm, the printing sensitivity may be decreased when conducting the thermal transfer printing, thus making it impossible to obtain satisfactory printing quality.
Now, the thermal transfer sheet of the second embodiment of the present invention for attaining the first object will be described in detail with reference to Figure 2. Figure 2 is a schematic partial cross sectional view illustrating the thermal transfer sheet of the aforementioned second embodiment of the present invention.
As shown in Figure 2, the thermal transfer sheet A2 of the second embodiment of the present invention comprises a substrate sheet 1, a release layer 3 formed on one surface of the substrate sheet 1, a colouring layer 2 formed on the release layer 3, an adhesive layer 4 formed on the colouring layer 2 and a back surface layer 5 formed on the other surface of the substrate sheet 1.
The substrate 1, the colouring layer 2, the release layer 3 and the adhesive layer 4 are identical with those of the thermal transfer sheet A1 of the first embodiment of the above-described present invention. The same reference numerals are given to these identical constitutional elements, and the description thereof is omitted.
The description of the back surface layer 5 will be given below.

[Back surface layer 5]

The back surface layer 5 is formed on the other surface of the substrate sheet 1 in order to prevent a thermal head from being stuck onto the other surface of the substrate sheet 1 and facilitate the smooth running of the thermal head thereon.
For forming the back surface layer 5, it is preferable to use a composition obtained by adding a lubricant, a surfactant, inorganic particles, organic particles and/or a pigment to a binder comprising a resin.
Representative examples of the resin to be used as a binder may include cellulosic resins such as ethyl cellulose, hydroxy-ethyl cellulose, hydroxy-propyl cellulose, methyl cellulose, cellulose acetate, cellulose butyl acetoacetate and nitrocellulose; vinyl resins such as polyvinyl alcohol, polyvinyl acetate, polyvinyl butylal, polyvinyl acetal, polyvinyl pyrrolidon, acrylic resin, polyacrylamide and acrylonitrile-styrene copolymer; polyester resins; polyurethane resins; and silicone-modified or fluorine-modified urethane resins.
There may preferably be used a bridged resin obtained by mixing any resin having several reactive groups, for example, hydroxyl groups, of the above-mentioned resin, with a crosslinking agent comprising polyisocyanate.
The back surface layer 5 can be formed on the other surface of the substrate sheet 1 by dissolving or dispersing materials in an appropriate solvent, which have been obtained by adding the above-mentioned lubricant, surfactant, inorganic particles, organic particles and/or pigment to the binder comprising the above-mentioned resin, to prepare a composition, applying the thus prepared composition to the other surface of the substrate 1 with the use of any one of the conventional means such as a gravure coater, a roll coater and a wire bar, and drying same.
The above-described thermal transfer sheets of the present invention may be used as a thermal transfer recording medium which is used for a thermal printer, a facsimile transmission apparatus or the like. In this case, a lead film is connected to the upstream end of the thermal transfer sheet relative to the travelling direction thereof. On the lead film, there may previously be printed (1) an indication including a description and/or descriptive drawings of matters to be attended to, of handling of the thermal transfer recording medium when charging this medium into a cassette or a printer, and/or (2) an indication having an arrow indicating a travelling direction of the thermal transfer sheet. In addition, the downstream end of the thermal transfer sheet relative to the travelling direction thereof may be connected to a feeding bobbin so as to wind the thermal transfer sheet into a coil around the feeding bobbin, and the forwarding end of the above-mentioned lead film may be connected to a receiving bobbin.

EXAMPLE

Now, the present invention will be described hereinbelow in more detail with reference to the following Example. In the description appearing hereinafter, part(s) and percentage (%) are part(s) by weight and weight percentage, respectively, unless otherwise noted specifically.
A thermal transfer sheet was prepared. A 4.5µ m thick polyethylene terephthalate film (Product name: "Lumirror" manufactured by TORAY Co. Ltd.) was used as a substrate sheet. On the one surface of the substrate, a back surface layer forming composition having the chemical composition described below was applied in a coating amount of 0.3 g/m² (based on solid content), and the resultant coating was dried to form a back surface layer.
Then, on the other surface of the substrate, a release layer forming composition having the chemical composition described below was applied in a coating amount of 0.3 to 0.5 g/m² (based on solid content) by a gravure coating method, and the resultant coating was dried to form a release layer.
Then, on three areas of the thus formed release layer which lay in a row in the width direction of the substrate, colouring layer forming compositions for yellow, magenta and cyan having the respective chemical compositions described below were applied, respectively, in a coating amount of 0.7 g/m² (based on solid content) by the gravure coating method, and the resultant coatings were dried at 70°C to form a colouring layer comprising rows of yellow, magenta and cyan on the same plane. An adhesive layer was formed on the heat fusible colouring ink layer by applying the adhesive layer forming composition having the chemical composition described below in a coating amount of 0.7 g/m² (based on solid content) by the gravure coating method, and the resultant coating was dried at 80°C.

The back surface layer forming composition, the release layer forming composition, the heat fusible colouring layer forming compositions and the adhesive layer forming composition had the following chemical compositions:

<Back surface layer forming composition>
Styrene-acrylonitrile copolymer	11 parts
Linear saturated polyester resin	0.3 parts
Zincstearylphoshpate	6 parts
Melamine resin powder	3 parts
Methyl ethyl keton	80 parts

<Release layer forming composition>
Carnauba wax ("WE-95" manufactured by KONISHI KABUSHIKI KAISHA)	10 parts
Water/isopropyl alcohol (wt. ration of 1/3)	30 parts

<Colouring layer forming composition (yellow)>
Benzimidazolone-mono-azo (Yellow 120)	8 parts
Benzimidazolone-mono-azo (Yellow 180)	8 parts
Cellulose-acetate-butylate resin (CAB)	8 parts
Dispersing agent	1.2 parts
Toluene.methyl ethyl ketone (wt.ratio of 1/1)	75 parts

<Colouring layer forming composition (magenta)>
2,9-dichloroquinacridone (Red 202)	6 parts
Benzimidazolone-mono-azo (red 176)	6 parts
Cellulose-acetate-butylate resin (CAB)	8 parts
Dispersing agent	1.2 parts
Toluene/methyl ethyl ketone (wt. ratio of 1.1)	79 parts

<Colouring layer forming composition (cyan)>
Phthalocyanine blue (15:4)	8 parts
Cellulose-acetate-butylate resin (CAB)	12 parts
Dispersing agent	0.7 parts
Toluene/methyl ethyl ketone (wt. ratio of 1/1)	80 parts

<Adhesive layer forming composition>
Polyester resin	36.8 parts
Carnauba wax No. 1	17.6 parts
Oxidised micro-crystalline wax	2.5 parts
Polyethylene wax	39.3 parts
Surfactant	3.7 parts

Then, for the resultant printed material, i.e., the samples for evaluation, the weatherproofing property was evaluated by the following method:

Light was irradiated from a xenon electronic lamp onto the surface of the sample with the use of a xenon fade meter manufactured by ATLAS Co. Ltd., and a decreased value of density according to its dose was measured by a reflection density measuring apparatus "MACBETH RD-914". Difference between the density values of the surface of the sample before and after the irradiation of the light was obtained, and the thus obtained value of difference was expressed in percentage for the evaluation of the weatherproofing property. The smaller value expressed in percentage meant an excellent weatherproofing property.
Evaluation results are shown in the Table below.

Yellow Magenta Cyan

Example 1% 2% 2%
As is clear from the TABLE, the sample of the Example had an excellent weatherproofing property in yellow, magenta and cyan, and especially in yellow and magenta.
In addition, the samples obtained by the thermal transfer sheet of the present invention had an excellent full colour tone reproduction property in an intermediate colour of red by a multiple printing of yellow and magenta, an intermediate colour of green by a multiple printing of yellow and cyan, an intermediate colour of violet by a multiple printing of magenta and cyan, and an intermediate colour of grey by a multiple printing of yellow, magenta and cyan.
According to the present invention as described in detail for attaining the first object, the use of the same kind of material in the release layer and the adhesive layer permits to improve a multiple printing property to provide a good printing quality without occurrence of void in a multiple printing portion, and makes it possible to manufacture a thermal transfer sheet excellent in full colour tone reproduction property.
Preferred materials according to the invention yield images having an excellent weatherproofing property, a printed material obtained from such a thermal transfer sheet can withstand use as an outdoor bill board for a long time of period, unlike a printed material obtained by the conventional thermal transfer sheet, which easily tends to fade, thus permitting the expanded use of the thermal transfer sheet, for example as a license plate for a vehicle such as a car, a road sign, or the like.

Claims

A thermal transfer sheet comprising a substrate sheet (1), a release layer (3) formed on one surface of said substrate sheet, a colouring layer (2) formed on said release layer and an adhesive layer (4) formed on said colouring layer, wherein:
said release layer and said adhesive layer contain a material of the same kind as one another and said colouring layer has multiple areas each consisting essentially of a respective colouring agent and resin, but without wax.
A thermal transfer sheet as claimed in Claim 1, wherein:
said same kind of material is carnauba wax.
A thermal transfer sheet as claimed in Claim 1, wherein:
said same kind of material is polyethylene wax.
A thermal transfer sheet as claimed in Claim 1, wherein:
said same kind of material is polyester resin.
A thermal transfer sheet as claimed in any preceding claim, wherein:
said colouring layer comprises a heat fusible colouring ink layer; and a benzimidazolone-mono-azo organic pigment is used as a yellow colouring agent in said heat fusible colouring ink layer, a mixture of quinacridone-mono-azo and benzimidazolone-mono-azo organic pigments is used as a magenta colouring agent in said heat fusible colouring ink layer, and a phthalocyanine organic pigment is used as a cyan colouring agent in said heat fusible colouring ink layer.